[uf-rest] AHAH and delay: "push" as "lazy pull"

[Dr. Ernie Prabhakar]

Hi Kevin,

Picking up this conversation, a friend just pointed me to:

http://www.clipcode.org/messaging/http_async_notif.html

This seems a pretty reasonable breakdown of when you can (or can't)  
use HTTP asynchronously.   Would you agree or disagree?

One more question:  if I as a client initiate an HTTP request and  
wait for the response, the only way to cancel that request is to  
close the entire connection -- which means that if I need out-of-band  
communication, I either need a second connection, or I keep tearing  
down and reestablishing communication.  Right?

-- Ernie P.

On Nov 28, 2005, at 3:37 PM, Kevin Marks wrote:

>>>> However, my hypothesis is that you can also implement "push" as  
>>>> "lazy pull."
>>>
>>> I'm pretty sure I've seen this hypothesis proven already.
>>
>> I know Rohit proposed it a while ago, as Roy  alludes to here:
>>
>> http://groups.yahoo.com/group/rest-discuss/message/5266
>>
>> I'm just not (yet) sure of how generally applicable that is
>
> The underlying 'pending http' model is widely used. In particular,  
> it is how DAAP (the iTunes sharing protocol) handles synchronising  
> playlist changes and detecting disconnects -(you need no traffic to  
> maintain a pending TCP connection, but you do get a teardown  
> message when it goes).
>
> I also helped Greg Elin build this kind of protocol for his network  
> chat application over XMLHTTPRequest.
>
> The key to doing it reliably is to use a sequence number in the  
> requests (not a timestamp, as that has potential skew and races),  
> so the server knows what you have already got. Also, you should  
> have exponential backoff and retry on timeouts and other dropped  
> connection errors.
>
> A simple chat app using this might be an interesting bit of sample  
> code.
>
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